Industrial coatings are used to protect the surface of a substrate against deterioration brought about by the action of light, humidity, wear, atmospheric oxygen, and other chemicals, and to impart the desired appearance such as colour, gloss, and surface structure. In many cases, such coatings are based on organic polymers which show good adhesion to the substrate and form a film free from defects such as pores or blisters. Film formation, also referred to as drying, is the transition of the coating composition applied to the solid state. The solid film can be formed from a solution by removal of solvent or from a dispersion by removal of the dispersing agent, or from a melt by cooling. In this case, and if no chemical reaction occurs, this is referred to as “physical drying”. In the so-called chemical drying, chemical reactions occur during film formation which lead to crosslinked macromolecules. Such crosslinking may be caused by chemical reaction of low molar mass molecules, oligomers or macromolecules between themselves, such as addition or condensation reactions, or radiation induced or thermally induced polymerisation, or by the action of added polyfunctional molecules, the so-called crosslinkers, which react with functional groups of polymers usually referred to as binder resins.
A well-known class of crosslinkers used in conjunction with binder resins having active hydrogen-containing reactive groups, such as hydroxyl and carboxyl groups, are the so-called amino resins, which are hydroxy functional adducts of aldehydes, generally formaldehyde, and organic amino compounds such as triazines, particularly preferably melamine, and urea or derivatives of these, the hydroxyl groups of which are usually at least partially etherified with lower alcohols such as methanol, and n- or iso-butanol. These crosslinkers suffer from the drawback that formaldehyde, inter alia, is liberated during the curing or crosslinking reaction. Emission of formaldehyde is environmentally undesirable. Additionally, many of these amino resins need temperatures typically of at least 80° C. to act as crosslinkers. Heating to such elevated temperatures is both time-consuming and energy-consuming.
In the PCT application WO2009/073836 A1, a process is disclosed for the preparation of crosslinkers based on reaction products of cyclic ureas and multifunctional aldehydes having at least two aldehyde groups which can be used in coating compositions comprising active hydrogen containing resins, such as hydroxy functional alkyd, acrylic, urethane or epoxy resins, and which coating compositions can be cured with such crosslinkers even at ambient temperature. The coatings prepared therewith showed good stability against solvents, and were not prone to yellowing. This process makes use of a multi-step reaction sequence where in the first step, the aldehyde component is mixed with an alcohol, and reacted under acidic conditions leading to formation of hemiacetals and acetals, and then in the second step, this mixture is reacted with a cyclic urea which may be preformed, or formed in situ. Depending on the reaction time, reaction conditions, and storage time in the first step, the hemiacetals and acetals may undergo oligomerisation, disproportionation and condensation reactions, leading to formation of a mixture of individual compounds such as mono- and diacetals of monomeric, dimeric or trimeric glyoxal, esters of glyoxylic acid, and glycolates. See S. Mahajani and M. M. Sharma in Organic Process Research and Development, 1997, No. 1, pages 97 to 105; and J. M. Kliegman and R. K. Barnes, J. Org. Chem., Vol. 38 (1973), No. 3, pages 556 et seq. The composition of this mixture has been found to be difficult to control. Owing to the fact that the aldehyde is converted to its acetal or hemiacetal, there is no possibility to form non-etherified adducts.
In the Japanese Patent Publication 53-044567, reaction of glyoxal with cyclic urea (2:1 mole ratio) in presence of a strong acid is disclosed. Our attempts at reacting glyoxal with ethylene urea (at a ratio of the amounts of substance of 1.2:1 mol/mol) in presence of a strong acid led to the formation of a rubbery gel-like solid product unusable for surface coating applications.